1. Field of the Invention
The present invention relates to methods and apparatus for cutting or milling a window in a cased borehole so that a secondary or deviated borehole can be drilled. More particularly, the invention relates to methods and apparatus for forming a window of optimal dimensions. Still more particularly, the invention relates to methods and apparatus for deviating a mill tool radially outwardly from an optimal cutting position to a location outside of the casing.
2. Description of the Related Art
It is common practice to use a whipstock and mill arrangement to help drill a deviated borehole from an existing earth borehole. The whipstock is set on the bottom of the existing earth borehole or anchored within the borehole. The whipstock has a ramped surface that is set in a predetermined position to guide a mill in a deviated manner so as to mill away a portion of the wellbore casing, thus forming a window in the steel casing of the borehole.
The typical whipstock presents a ramped surface which has a substantially uniform slope such as three degrees from the vertical. Thus, the mill tool is normally urged outwardly at a constant rate until it is fully outside of the casing. As the mill moves downward within the borehole, the ramped surface of the whipstock urges the mill radially outwardly so that the cutting surface of the mill engages the inner surface of the casing. As this engagement begins to cut into the casing, the casing is worn away and then cut through, thus beginning the upper end of the window. The ramp of the whipstock then causes further deviation of the mill, causing the mill to move downwardly and radially outward through the casing itself. Thus, a longitudinal window is cut through the casing. Ultimately, the whipstock's ramped surface urges the mill radially outwardly to the extent that it is located entirely outside of the wellbore bore casing. Once this occurs, the mill ceases cutting the window. This traditional cutting technique results in an upside-down “teardrop” shaped window which has a section of maximum width located close to the top of the window. From this section of maximum width, the width of the window decreases and the window tapers as the lower portion of the window is approached. An example of such a window is shown in prior art FIG. 1.
Once the window is cut in the manner described above, a deviated borehole is then cut using a point of entry that is proximate the teardrop-shaped window. Unfortunately, the teardrop shape of the window can impede the ability to drill the deviated borehole. Specifically, as the window narrows, the metal portion of the casing interferes with the ability to drill, place liners and so forth.
Thus, a need exists for methods and devices that can be employed to form a window in a casing wall that has optimum or near optimum dimensions so that subsequent directional drilling efforts are not hindered.